1. Field of the Invention
The present invention relates to a backplane-based communications system and, more particularly, to a high-speed, open collector, shared backplane-based communications system with increased noise immunity when the backplane is lightly-loaded.
2. Description of the Related Art
A backplane-based communications system is a system that electrically connects together a number of electronics cards, such as XDSL line cards, via a multi-drop transmission line (MDTL) that runs through the backplane. Each of the electronics cards includes transceivers that receive information from, and transmit information to, the MDTL in accordance with the rules defined by a communications protocol.
One common communications protocol is the asynchronous transfer mode (ATM) protocol. The ATM protocol defines the rules for transferring data across a network in 53-byte cells that include a 48-byte data field and a 5-byte header. Another communications protocol is the synchronous optical network (SONET) protocol. The SONET protocol defines the rules for outputting serial data streams of various rates, such as 155.52 Mbps.
FIG. 1A shows a perspective view that illustrates a prior-art, backplane-based communications system 100. FIG. 1B shows a schematic diagram that illustrates a portion of communications system 100. As shown in FIGS. 1A–1B, communications system 100 includes a backplane 110, and a number of electronics cards 112 that are plugged into backplane 110. In the FIG. 1A example, two electronics cards 112 are plugged into backplane 110, one at each end, while in the FIG. 1B example, 16 electronics cards 112 are plugged into backplane 110, filling backplane 110.
Backplane 110 has a first MDTL 114 that represents a first number of metal lines, and a second MDTL 116 (not shown in FIG. 1B) that represents a second number of metal lines. First MDTL 114 can include, for example, 16 metal lines (dual byte wide), while second MDTL 116 can include, for example, 34 metal lines.
In addition, backplane 110 has a number of connecters 118 that are connected to the first and second MDTLs 114 and 116. The connectors 118 are uniformly distributed along the length of the first and second MDTLs 114 and 116 to have, for example, a 2.54 cm (one inch) spacing.
As further shown in FIG. 1, each electronics card 112 has a receiver 120 and a transmitter 122 that is electrically connected to first MDTL 114. Receiver 120 receives ATM data from, and transmitter 122 transmits ATM data to, first MDTL 114 utilizing, for example, backplane transceiver logic (BTL). The BTL can run at, for example, 30 MHz, or 33.3 nS/word, yielding 480 Mbps of ATM traffic bandwidth (1(33.3 nS/16 bits per word)).
Optionally, each electronics card 112 could include a transceiver 124 (not shown in FIG. 1B) that is electrically connected to second MDTL 116. Transceiver 124 could receive SONET-framed serial data from, and transmit SONET-framed serial data to, second MDTL 116 utilizing, for example, 32 traces for data and 2 traces for control signals, yielding 4.976 Gbps (155.52 Mbps per trace*32 traces). As a result, communications system 100 optionally provides two backplane buses: an ATM-based bus and a SONET-based bus.
One drawback of communications system 100 is that the cost of terminating high-speed serial SONET streams at 155 Mbps on individual electronics cards, such as cards 112, is very high. As a result, there is a need for a communications system that provides a data rate of approximately 150–155 Mbps across a backplane bus that is less expensive than terminating a SONET stream on an individual electronics card.
Another drawback of communications system 100 is that first and second MDTLs 114 and 116 have impedance mismatches that vary with the load (the number of electronics cards 112 that are connected to the first and second MDTLs 114 and 116). As described below, the impedance mismatches lead to standing waves that may limit the maximum operating frequency fop when an MDTL is lightly-loaded.
An inherit property of MDTLs is an intentional mismatch between the characteristic impedance Zo of the transmission line, which varies as a function of the load, and the impedance of the terminating network Zt. The impedance mismatch is designed such that, under full loading (where an electronics card 112 is plugged into each connector 118), the transmission line characteristic impedance Zo matches the network impedance Zt (i.e., there is no impedance mismatch when an electronics card 112 is plugged into each connector 118).
However, under light loading (where a number of the connectors 118 are empty), impedance mismatch causes reflection waveforms that form an interference pattern. Under certain circumstance, interference patterns form a phenomenon known as standing waves which, in turn, cause severe attenuations of the incident waveforms.
The severity of the attenuation is proportional to the magnitude of the mismatch between the transmission line and network impedances Zo and Zt. If the attenuation is severe enough, the result is communication system failure. As a result, the conductors have to be properly terminated to preserve signal integrity.
To illustrate the variation of the transmission line characteristic impedance Zo as a function of the load, consider the case of a lightly-loaded, lossless MDTL that has only two electronics cards attached to it, one card at each end of the MDTL, such as shown in FIG. 1A. In this case, the MDTL is a point-to-point transmission line. For a point-to-point transmission line, the transmission line characteristic impedance Zo is defined by equation EQ. 1 as:
                                          Z            O                    =                                    L              C                                      ,                            EQ        .                                  ⁢        1            while the propagation delay τpd is defined by equation EQ. 2 as:τpd=√{square root over (LC)}  EQ. 2where L and C are the line inductance and capacitance per unit length, respectively. In addition, the transmission line characteristic impedance Zo is real (i.e. behaves like a resister) and is only a function of the transmission line geometry. Hence, a lossless transmission line is completely specified by its characteristic impedance Zo and propagation delay τpd.
Now consider the case where all the loads are attached to the MDTL. The loads introduce a distributed capacitive loading per unit length Cd. Thus, the MDTL has a new value for both characteristic impedance Zo′ and propagation delay τpd′. These new values are calculated according to equations EQs. 3, 4, and 5 as:
                                          Zo            ′                    =                                    L                              C                +                Cd                                                    ,                            EQ        .                                  ⁢        3                                                      τ            ⁢                                                  ⁢                          pd              ′                                =                                    L              ⁡                              (                                  C                  +                  Cd                                )                                                    ,                                  ⁢        and                            EQ        .                                  ⁢        4                                Cd        =                  NCl          H                                    EQ        .                                  ⁢        5            where Cl is the load capacitance, N is number of loads, and H is the length of the transmission line. Hence, as loads are added to a lightly-loaded transmission line, the transmission line impedance Zo′ reduces and the propagation delay τpd′ increases.
For example, assume that an MDTL has 16 loads that are each uniformly spaced 2.54 cm (one inch) apart. The loads are represented by their total capacitance Cl (i.e. IC I/O pin, vias, connecters, and wires capacitance). A reasonable estimation of Cl for a typical high-speed, open collector based MDTL is 12 pf. Furthermore, assume that the MDTL is an FR4 stripline with a transmission line impedance Zo=60 Ω.
The capacitance C and inductance L can be calculated from EQs. 1 and 2 as shown in EQs. 6 and 7 as:
                              C          =                                    τ              ⁢                                                          ⁢              pd                        Zo                          ,        and                            EQ        .                                  ⁢        6                                L        =                              Zo            2                    ⁢                      C            .                                              EQ        .                                  ⁢        7            
As a result, C=3 pf and L=10.8 nH. Next, Cd is calculated from equation 5 which gives us Cd=12 pf. Then, from equations EQs. 3 and 4:
                                                        Zo              ′                        =                                                                                10.8                    ⁢                                                                                  ⁢                                          ⅇ                                              -                        9                                                                                                  (                                                                  3                        ⁢                                                                                                  ⁢                                                  ⅇ                                                      -                            12                                                                                              +                                              12                        ⁢                                                                                                  ⁢                                                  ⅇ                                                      -                            12                                                                                                                )                                                              ≈                              27                ⁢                                                                  ⁢                Ω                                              ,          and                ⁢                                                                    τ          ⁢                                          ⁢                      pd            ′                          =                                            10.8              ⁢                                                          ⁢                                                ⅇ                                      -                    9                                                  ⁡                                  (                                                            3                      ⁢                                                                                          ⁢                                              ⅇ                                                  -                          12                                                                                      +                                          12                      ⁢                                                                                          ⁢                                              ⅇ                                                  -                          12                                                                                                      )                                                              ≈                      400            ⁢                                                  ⁢                          ps              .                                          
From the above, under a full load, the transmission line characteristic impedance Zo drops from 60 Ω to 27 Ω and τpd slows down from 180 ps/in to 400 ps/in.
The termination impedance Zt must then be set to 27 Ω to match the transmission line characteristic impedance Zo′ under a full load. As a consequence, at light loading, the network termination impedance Zt is mismatched with the transmission line characteristic impedance Zo and reflections are present in the transmission line.
The impedance mismatches lead to reflections which lead to standing waves which, in turn, limit the maximum operating frequency fop when the MDTL is lightly-loaded. There are many factors that limit how fast a MDTL may operate (i.e. maximum fop). Some factors include IC propagation delay, time of flight, clock to data skew, and setup and hold times.
Standing waves become an issue in long MDTLs that contain sizable reflections compared to the amplitude of the incident wave. To avoid standing-wave problems in an MDTL, the length of the MDTL should be comfortably shorter than one-fourth the wavelength (λ/4) of the operating frequency fop. If the above condition is not satisfied, then the maximum incident wave attenuation must be evaluated to ensure that is acceptable.
For example, a typical 48.26 cm (nineteen inch) telecommunication equipment rack has a usable backplane (MDTL) length of about 43.1 cm (17 inches). In addition, assume a propagation delay τpd of 180 pS/2.54 cm (180 pS/inch). The frequency fq at which 43.1 cm (17 inches) (the length of the MDTL) matches (λ/4) is found using equation EQ. 8:
                    fq        =                  1                      4            ⁢            τ            ⁢                                                  ⁢                          pd              ⁡                              (                Ltl                )                                                                        EQ        .                                  ⁢        8            where Ltl is the length of the MDTL. Substituting in the values of the example produces:
  fq  =            1              4        ⁢                  (                      180            ⁢                                                  ⁢                          ⅇ                              -                12                                              )                ⁢                  (          17          )                      ≈          81.7      ⁢                          ⁢              MHz        .            
Thus, when lightly loaded, the MDTL of the present example has a maximum operating frequency fop of approximately 82 MHz. If the maximum operating frequency fop of communications system 100 is substantially less than fq, such as 30 MHz, then standing waves present little problem. On the other hand, if the maximum operating frequency fop is equal to fq, then communications system 100 has reduced noise immunity, and may not be suitable for many applications.
To perform a simulation of an MDTL to evaluate the noise immunity at 82 MHz, assume a backplane length of about 43.1 cm (17 inches), 2 loads, a transmission line characteristic impedance Zo of 60 Ω, a transmission line characteristic impedance Zo′ of 27 Ω, a propagation delay τpd of 180 pS/2.54 cm (180 pS/inch), a propagation delay τpd′ of 400 pS/2.54 cm (400 pS/inch), a termination line impedance Zt of 27 Ω, and a source impedance Zs of 27 Ω.
Further assume that the drivers that are connected to the MDTL are open collectors, which require a pull-up resister to a 2.1 v termination voltage. Thus, when all the drivers are off, the steady state voltage on the MDTL is 2.1 v. On the other hand, when one of the drivers turns on, the steady state voltage is 1 v.
Thus, the waveform has an amplitude swing of 1.1 v, a minimum input high threshold Vih=1.62 v, and a maximum input low thresh hold Vil=1.47 v. (These values are typical of transceiver model number FB1653 manufactured by Texas Instruments.) As a result, there is not much room for noise in this system.
In addition, to perform the simulation, the magnitude ρl of the waveform after first being reflected back from the receiver (the load reflection coefficient of the MDTL), and the magnitude ρs of the waveform after first being reflected back from the driver ρs (one round trip after first being driven) (the source reflection coefficient) of the MDTL are calculated. When the driver turns off (i.e., the wave transitions from low to high), the magnitude of the reflected wave at the receiver ρl and ρs are:
                                          ρ            ⁢                                                  ⁢            l                    =                                                    Zl                -                Zo                                            Zl                +                Zo                                      =                                                            27                  -                  60                                                  27                  +                  60                                            ≈                              -                0.38                                                    ,                                  ⁢        and                                                                    ρ              ⁢                                                          ⁢              s                        =                                                            Zs                  -                  Zo                                                  Zs                  +                  Zo                                            =                                                                    27                    -                    60                                                        27                    +                    60                                                  ≈                                  -                  0.38                                                              ,                ⁢                                      where the product of ρlρs=0.144. This product means that the reflections from the low to high transitions attenuate rapidly. Further, the initial wave is also attenuated by the voltage divider of the source impedance of the driver Zs and the transmission line characteristic impedance Zo.
When the driver turns on (i.e., the wave transitions from high to low), the low impedance of the driver is now engaged and ρl and ρs become:
                                          ρ            ⁢                                                  ⁢            l                    =                                                    Zl                -                Zo                                            Zl                +                Zo                                      =                                                            27                  -                  60                                                  27                  +                  60                                            ≈                              -                0.38                                                    ,                                  ⁢        and                                                      ρ            ⁢                                                  ⁢            s                    =                                                    Zs                -                Zo                                            Zs                +                Zo                                      =                                                            0                  -                  60                                                  0                  +                  60                                            ≈                              -                                  1.0                  .                                                                    ⁢                                      
The product of ρlρs=0.38. Thus, the reflections from the high to low transition are larger due to the low impedance of the driver.
FIG. 1C shows an Hspice simulation of communications system 100 that illustrates the waveform along the length of MDTL 114. The simulation is based on the above assumptions and calculations using an operating frequency fop=82 MHz. As shown in FIG. 1C, the amplitude of the waveform v(b1) at point by (one end of MDTL 114 shown in FIG. 1B) decreases as the waveform moves to point b3 (the other end of MDTL 114 shown in FIG. 1B), as shown by waveform v(b3) taken at point b3.
FIG. 1D shows an Hspice simulation that illustrates the waveform at the end of MDTL 114 of communications system 100 where the worst attenuation of the waveform is expected. As shown in FIG. 1D, the waveform v(b3) at point b3 rises almost to its full high steady state voltage (2.1 v). This confirms that reflections from the low to the high transitions are not significant.
On the other hand, FIG. 1D shows that the low state voltage is off by 323 mv. This is a significant loss for a 1.1 v amplitude waveform. Consequently, the low threshold noise immunity has reduced from 470 mv to about 150 mv. Considering other system noise sources such as thermal, power supply, and crosstalk noise, 150 mv noise immunity may not be acceptable. Further, the high threshold noise immunity is about 332 mv, 2.2 times the low threshold noise immunity. Thus, unless communications system 100 can accept these constraints, communications system 100 can not run at 82 MHz.
Thus, in order to obtain a reasonable noise immunity, the maximum operating frequency fop must be substantially reduced which, in turn, substantially reduces the data rate, or the length of MDTL 114 must be reduced which, in turn, reduces the number of loads that can be connected to MDTL 114.
As a result, there is a need for a backplane-based communications system that provides a high data rate when the MDTL is lightly loaded without reducing the number of loads that can be connected to MDTL 114.